1. Field of the Invention
The invention concerns a case or box comprising a supporting plate carrying a circuit or an electronic component, covered by a cap, the carrier also including a capacitor.
2. Discussion of Background
In the present specification, a component is understood to be any active or passive component, discrete or integrated, as well as any electronic circuit (hybrid circuit, for instance) likely to be placed in the box.
As is well known, it frequently happens that a component or electronic circuit requires one or several auxiliary capacitors, especially for decoupling, which cannot be integrated onto the same substrate as the component in question. The capacitors are therefore added onto the printed circuit card which bears the component and its box, which occupies a considerable area of the surface in proportion to that occupied by the component. This is in direct contrast to the need to increase the density presently available.
In order to reduce this difficulty, it is known, in accordance with the French Patent Application No. 79 11852, in the name of THOMAS-CSF, that the supporting plate of the box may be used to create a capacitor. To this end, a ceramic plate is selected which constitutes the dielectric element of the capacitor overlayed by the necessary metallizations to provide the armatures of this capacitor. However, this solution is unsuitable for certain applications, notably those which use power components or have a high integration factor. Indeed, the supporting plate of a box presents special thermal properties, including a coefficient of thermal expansion which is as close as possible to that of the semiconductor material employed, this most frequently being silicon, which generally results in alumina being chosen for the supporting plate. Another property which the supporting plate must have is a good thermal conduction for discharging the calories. These various requirements are not necessarily compatible with the embodiment of a capacitor, mainly as regards the dielectric constant of the supporting plate and the appearance of disturbing capacitive couplings between the circuit connections and those of the capacitor.
A second structure has been proposed and described in the French Patent Application 82 11402 in the name of CIMSA, in which the supporting plate presents a structure which is thermally optimized, the capacitive function being embodied on or inside the cap of the carrier. This structure, if it enables the previous problems to be avoided, does however lead, in certain cases, to difficulties in realization due to the fact that, at the time of fixing the cap onto the supporting plate, it is necessary to both seal the cap on the supporting plate together and make the electrical connections of the capacitor with the remainder of the circuit. In addition and is as well known, present components tend to process signals whose voltage is lower and lower with faster and faster response times. Consequently these circuits, it is necessary to reduce as far as possible the impedance of component-capacitor interconnections which, it will be recalled, is proportional to the square root of the ratio L/C. For this purpose, it is necessary to reduce the inductance L of these connections, which is carried out by reducing their length. For certain applications, it may be desirable to further reduce this impedance relatively to that which is obtained for a structure of the type described in the previously quoted patent application 82 11402.